In order to be able to achieve constant speedkeeping for a motor-vehicle on downhill slopes, regulation of the vehicle's brakes towards a target speed selected by the driver is already known.
In the case of heavier motor-vehicles, there are used in this connection various types of auxiliary brakes over and above the vehicle's ordinary wheel brakes. These auxiliary brakes can be designed in such cases with a forced cooling system which allows for prolonged application of the auxiliary brakes without the vehicle's main brakes being used. As a result, the main brakes, which conventionally comprise friction brakes of the disc brake or drum brake type, are prevented from overheating, with the associated risk of brake failure. With the use of auxiliary brakes, increased operating safety is thus obtained in the braking system, since the auxiliary brakes are advantageously used for constant speedkeeping purposes and for moderate decelerations, whilst the vehicle's main brakes are saved for violent brakings provoked by unexpected disturbances in the traffic flow.
Sytems for constant speedkeeping on downhill slopes by the regulation of auxiliary brakes are previously known through, for example, patent specifications U.S. Pat. Nos. 4,485,444 4,477,124 and 4,462,479, in which systems built up of logic circuits, for example in the form of a microcomputer, regulate the brakes towards a target speed set by the driver.
The known constant speedkeeping systems are actuated by a driver-activatable control system which is used exclusively for this purpose. This control system can, for example, be disposed upon the steering wheel console in the form of a lever or a rotary control or, in certain cases, a foot-operated control over and above the vehicle's ordinary foot pedals for acceleratoar, brakes and clutch. Constant speedkeeping systems have also been proposed, actuated through some forms of push-bottom programming by the driver.